Semiconductor lasers have a variety of applications including communication systems and consumer electronics. Generally, semiconductor lasers may be categorized as edge-emitting lasers or surface emitting lasers (SELs). An edge-emitting laser emits radiation parallel to a surface of the semiconductor wafer or die, while a SEL emits radiation substantially perpendicular to the surface. One common type of SEL is a vertical cavity SEL (VCSEL). A VCSEL includes a gain region within a resonant cavity with reflectors parallel to a substrate surface and having a surface aperture to emit light from the resonant cavity.
There are two main techniques for modulating a signal onto an optical carrier wave emitted from a semiconductor laser—direct modulation and external optical modulation. Direct modulation encodes the optical carrier wave with a signal by directly modulating the drive current applied to the gain region of the semiconductor laser. The bandwidths achieved by direct modulation are limited due to the finite relaxation oscillation time of an excited state electron within the gain region. This finite relaxation oscillation time can result in inter-symbol interference (ISI) between adjacent clock cycles. With external optical modulation, the semiconductor laser emits a continuous wave (CW) carrier, which is modulated by an external optical modulator (EOM). EOMs are typically distinct entities from the CW carrier source and therefore more expensive to manufacture than directly modulated lasers, but are capable of achieving higher modulation bandwidths.
Generally, EOMs may be categorized as electro-refraction modulators and electro-absorption modulators. Electro-refraction modulators rely on changes in the index of refraction of a material induced by an applied electric field to modulate the proportion of light through the modulator (for example, Mach-Zehnder interferometer). Electro-absorption modulators achieve the desired light modulation by modifying the light absorbing properties of a material with an electric field.
Notwithstanding the technique employed for modulating a signal, modulator feedback can negatively effect operation of the semiconductor laser. For example, high modulation frequency, vertical cavity surface-emitting lasers (VCSELs) and dense VCSEL arrays are expected to change the paradigm for short-range interconnections, silicon integrated circuit inputs/outputs (and perhaps long interchip) to module and board-level connections. The bandwidth of current-modulated laser diodes is limited to about 20 Gb/s by intrinsic dynamic processes in the gain medium, such as optoelectronic relaxation phenomenon and carrier transport. The bandwidth can be significantly increased by external modulation as in telecommunications applications. To prevent a high-frequency optical feedback from the modulator to the laser, resulting in parasitic amplitude modulation, frequency shift, unstable operation, excessive noise, all limiting the modulation bandwidth, an optical isolator is typically inserted between the laser and the modulator sections.